1. Field of the Invention
This invention relates to a passive optical part for optical communications system which transmits bidirectionally optical signals of two wavelengths between ports simultaneously. Light source devices, for example, light emitting diodes (LEDs) or semiconductor laser diodes (LDs) and photodetectors, for example, photodiodes(PDs) or avalanche photodiodes (APDs) are active optoelectronic parts which are indispensable for building optical communications networks.
2. Description of Related Art
This invention does not propose such an active optoelectronic device but proposes a new passive optical device, that is, a xe2x80x9csubmountxe2x80x9d with a filtering function. The submount is a chip base used for mounting a device chip which requires insulation from a package. The submount is an intermediate passive part between a chip and a package. A main purpose of the submount is to insulating the device from the package. Some devices need no submount (chip base). For example, an independent transistor stored in a metal package has a collector which is directly in contact with the package without a chip base. An independent diode kept in a metallic case is also directly mounted upon the metallic case without submount, since the metallic case is either an anode or a cathode.
Optoelectronic device chips of laser diodes (LDs), photodiodes (PDs) and light emitting diodes (LEDs) are diodes having an anode and a cathode. Unlike the independent metal-case stored diodes or transistors, some reasons forbid the optoelectronic device chips from adhering directly to the package. A package may include a monitoring photodiode (PD), an amplifier IC besides the laser diode (LD) or the photodiode (PD). The coexistence of various device chips prohibit the bottom electrodes (cathode in many cases) of PD chips, LED chips or LD chips from being in direct contact to the metallic package (the ground level). The bottom electrodes of the device chips should be electrically insulated from the package ground level. The xe2x80x9csubmountxe2x80x9d is an insulating board which should be inserted between the chip bottom and the package ground level. Besides the insulation, the submount has other roles of facilitating thermal diffusion, harmonizing the thermal expansion coefficients between the chip and the package or adjusting the height of the chip. In the case of enhancing thermal diffusion, regulating the thermal expansion coefficients or controlling the chip height, the submount need not be an insulator. In the case of the devices stored in metallic packages, the submount is a passive, intermediate chip base which plays the role of keeping a device chip upon a package. The submount has metallized layers formed on both surfaces for the sake of bonding.
Recently planar lightguide circuit (PLC) type transmitting/receiving modules (LD/PD modules) attract attentions instead of the prior discrete type modules which have independent PD, LD, AMP or ICs assembled on print circuit boards. The planar lightguide circuit (PLC) type device contains a silicon bench (Si-platform), device chips, a leadframe and a resin package. The PLC module is made by fixing the device chips upon the silicon bench, fitting the silicon bench on a central part of the leadframe and enclosing the silicon bench and the leadframe with a resin. The leadframe is a thin metal plate having a central part and a number of lead pins.
The PLC type modules never fit device (PD, LD) chips directly upon the leadframe but upon the silicon bench. Sometimes the device chips are directly fitted upon the silicon bench. Other times, rectangle thin plates are inserted between the chips and the silicon bench for adjusting the height of the chips. The rectangle plates for adjusting the height are also called xe2x80x9csubmountxe2x80x9d. The submount (chip base) has a wider concept than the insulating base. The submount is a passive optical part which is held between the silicon bench and the chip. Then, the object of the invention is called a xe2x80x9csubmountxe2x80x9d or xe2x80x9coptical partxe2x80x9d which intervenes between a case and a chip.
The submount or chip base is usually a rectangular intervening plate which is directly fitted upon a case (package, silicon-bench or so) for mounting a chip thereupon. The submount (chip base) is a rectangular opaque insulator having a metallized top surface and a metallized bottom surface. The metallized layers are made by plating, evaporating or printing a metal. Bonding the submount upon a case and die-bonding a chip upon the submount require the metallization on both surfaces.
The submount basic material is opaque and the metallized layers are also opaque. The chip base is triply opaque. There has been no transparent submount. No device chip on the submount has required light entrance via the bottom from the case. High heat conductivity is also one of desired properties of the chip base (submount) for removing heat from the chip rapidly. Ceramics or other dielectric materials are chosen as a material for making submounts. Prevailing submounts are made from alumina (Al2O3). Alumina excels in rigidity, chemical stability, electrical insulation and inexpensiveness. Alumina is also opaque. There has been no requirement for transparent submounts. There is actually no transparent submount. Prevalent submounts are simple rectangular insulating plates with an m/i/m layered structure, where xe2x80x9cmxe2x80x9d means a metal and xe2x80x9cixe2x80x9d means an insulator.
However, chip bases with an opening are seldom used for mounting a bottom incidence type photodiode upon a silicon bench of a PLC type module.
FIG. 1 shows a proposed PLC type PD portion in an LD/PD module which receives 1.55 xcexcm wavelength light and transmits 1.3 xcexcm wavelength light. The PD portion is fabricated upon a flat silicon bench 1. A V-groove 2 is dug from a front end to an intermediate point upon the silicon bench 1 in a longitudinal direction. A PD chip 3 is fitted upon the silicon bench 1 above a rear end of the V-groove 2. The PD chip 3 is a bottom incidence type PD which has a dielectric multilayered filter 5 on the bottom surface and an annular n-electrode 6 enclosing the filter on the bottom. The PD chip has a light receiving part 4 with a p-type region at the top. A frame-shaped rectangular submount 7 is soldered on a metallized pattern on the silicon bench 1. The bottom annular n-electrode 6 of the PD 3 is soldered on the frame-shaped submount 7 having an opening. An optical fiber 8 is fitted in the V-groove 2 with an adhesive. A mirror 9 is formed at the rear end of the V-groove 2. The signal light going out of the optical fiber 8 is reflected by the mirror 9, is turned upward and is introduced into the PD via the dielectric multilayered filter 5. The signal light arrives at the light receiving part 4 and produces photocurrent.
FIG. 1 shows only the PD portion. The LD/PD module contains an LD module (not shown in the figure) besides the device of FIG. 1. The LD module and the PD module are separated by a WDM (wavelength division multiplexer) filter with wavelength selectivity. In addition to the WDM, the PD has the dielectric multilayered filter 5 on the bottom. The transmitting wavelength is 1.3 xcexcm and the receiving wavelength is 1.55 xcexcm. The WDM separates the 1.3 xcexcm sending light and the 1.55 xcexcm receiving light. However, the extinction ratio of the WDM filter is still too poor to forbid a part of the 1.3 xcexcm light from leaking into the PD as stray light. The stray light would induce optical crosstalk in the PD module. The multilayered filter 5 is added to the PD for supplementing the WDM filter. Conventional InP PD has sensitivity to both the 1.55 xcexcm and the 1.3 xcexcm wavelengths. The reason why the conventional InP PD has sensitivity for both 1.55 xcexcm and 1.3 xcexcm.
FIG. 2 shows a section of a conventional InP PD equipped in PD modules. FIG. 2 shows a top incidence type PD having the same layer structure as the bottom incidence type PD. The top incidence type PD differs only on the shape of the electrodes from the bottom incidence type PD.
An n-InP buffer layer 13, an n-InGaAs light receiving layer 14 and an n-InP window layer 15 are epitaxially grown on an n-InP substrate 12 in series. A Zn-diffusion region (p-region) 16 is produced at a center of the top part by diffusing Zn atoms thermally. An annular p-electrode 17 is formed on an outer portion of the Zn-diffusion region 16. The central top aperture enclosed by the annular p-electrode is a opening through which signal light go into the PD. An antireflection film 18 is formed on the central aperture. The antireflection film is produced by piling by turns two kinds of dielectric materials having different refractive index and different thicknesses. The periphery of the chip is coated with a passivation film 19 for protecting revealing ends of the pn-junction. An overall n-electrode 20 is formed on the bottom of the n-InP substrate 12. A reverse bias is applied to the electrodes for detecting light. The reverse bias means an application of a positive voltage to the n-electrode 20 and a negative voltage to the p-electrode 17.
The conventional InP photodiode has a wide sensitivity range since the PD has an InGaAs light receiving layer. FIG. 3 shows a graph showing the sensitivity curve PQR of the InGaAs PD as a function of wavelength. A rise (P) at 0.95 xcexcm corresponds to the band gap wavelength of the InP window layer 15. A fall (R) at 1.67 xcexcm corresponds to the band gap wavelength of the InGaAs light receiving layer 14. An intermediate range Q between P and R denotes nearly uniform sensitivity. The 1.3 xcexcm and 1.55 xcexcm wavelengths are included within the wide range between P(InP band gap) and R(InGaAs band gap). Thus, the conventional InP photodiodes have enough sensitivity to both the 1.31 xcexcm light and 1.55 xcexcm light. The PD senses the 1.3 xcexcm transmitting light which induces the crosstalk.
In the LD/PD module, the dielectric multilayered filter 5 is added to the entrance of the PD for supplementing the insufficient extinction ratio of the WDM filter. The leakage of the transmitting 1.31 xcexcm to the PD is doubly killed by the WDM and the multilayer filter 5.
FIG. 1 shows the PD module having the apertured submount 7 and a 1.31 xcexcm cutting the dielectric multilayered filter 5. The aperture of the submount 7 coincides with the 1.3 xcexcm cutting filter. The submount aims at adjusting the height of the PD chip for increasing light power entering the PD. The submount 7 is dispensable, since the PD requires no insulation from the package.
Drawbacks accompany the prior module containing a couple of the 1.3 xcexcm reflecting filter and the apertured submount. A window should be perforated on the submount made of ceramics. Mechanical drilling on the submount should require an increase of cost of making the PD module. One problem is the additional cost induced by boring the aperture. Another difficulty is caused from problems of the multilayered filter of admitting 1.55 xcexcm (necessary light) but for prohibiting the 1.3 xcexcm (unnecessary light) from passing.
One problem is a strong incident angle dependence of the power of eliminating unnecessary light. The designed extinction ratio can be obtained only by orthogonal incidence (90 degrees) to the multilayered filter. The multilayered filter allows a part of unnecessary light to invade into the filter when the light shoots the filter at a slanting angle (not 90 degrees). Slanting incidence necessary light is partially reflected by the multilayered filter. The slanting incidence decreases the extinction ratio.
Another problem of the dielectric multilayered filter is that the mode of eliminating light is reflection. Since the unnecessary light is not absorbed but reflected, the unnecessary light survives and tries to enter the filter again. The multilayered filter having a number of mutually piled different films of different refractive indexes n1 and n2 has the wavelength selectivity of admitting necessary wavelength and reflecting unnecessary wavelength light.
The complex refractive indexes of the multilayers have no imaginary part but have real parts. The real part of the complex refractive index determines the mode of refraction. The imaginary part of the complex refractive index determines absorption or attenuation. The dielectric multilayers only rely upon the real part of the refractive index of the piled layers.
The dielectric multilayered filter excludes unnecessary light not by absorption but by reflection. The multilayered filter repulses orthogonally incidence unnecessary light by reflection. Once reflected unnecessary light survives in the package. The unnecessary light is repeatedly reflected by the parts in the package. Randomly scattered light in the package is called xe2x80x9cstray lightxe2x80x9d. Sometimes unnecessary scattered light penetrates the filter and invades into the PD at slanting angles. The filter cannot repulse slanting unnecessary light. The stray unnecessary light induces serious crosstalk in the PD.
The use of the multilayered filter is still a defective method of eliminating unnecessary light. Crosstalk is a problem induced from imperfection of the WDM filters even in a PD module. The crosstalk induces a more important problem in a LD/PD module which contains an LD and a PD at a short distance in a narrow space. The LD emits strong light signal of xcex1. The PD should not sense the xcex1 light which is noise for the PD.
FIG. 4 and FIG. 5 show prior art LD/PD modules. The LD/PD module adopts a WDM filter 21 made of a glass block. Any types of WDM is available for the LD/PD module of FIG. 4. The glass block WDM is made by preparing two isosceles glass columns 22 and 23, evaporating a dielectric multilayered mirror 24 on a slanting wall of one of the isosceles glass columns 22 and 23 and joining two glass blocks on the slanting walls. The dielectric multilayered mirror has a function of allowing 45xc2x0 incidence transmitting light xcex1 to pass straight and reflecting 45xc2x0 incidence receiving light xcex2 at a 45 degree incidence/reflection angle. An LD 25, a PD 26 and a fiber 27 are placed at three radial positions around the WDM 21. Transmitting light xcex1 emitted from the LD 25 makes its straight way in the glass WDM 21, passes the mirror 24 without reflection and goes into the fiber 27. On the contrary, the receiving light xcex2 propagating in the fiber 27 is introduced into the WDM 21. The slanting mirror 24 reflects the xcex2 light at a 45 degree incidence/reflection angle. The reflected xcex2 goes into the PD 26. The ratio of selective reflection or selective admission of necessary light to unnecessary of a WDM light is called an extinction ratio. Current high quality WDMs have a 1:1000 extinction rate at the highest. A small portion of xcex1 of the LD 25 leaks into the PD via the WDM 21, which causes crosstalk. The LD/PD module separated the path of the PD from the path for the LD. The separation of the paths is favorable for suppressing crosstalk from the LD to the PD.
FIG. 5 shows another LD/PD module which aligns a WDM, a PD and an LD along a straight line. A housing 30 contains a silicon bench (not shown in the figure) having a V-groove. An optical fiber 31 is fitted in the V-groove. The LD 32 is fitted at a rear end of the fiber on the bench. The WDM filter 34 is inserted into an oblique slit cutting slantingly the fiber 31. The PD 33 of a bottom incidence type is mounted above the WDM 34 on the silicon bench.
The LD 32 makes transmitting light of a wavelength xcex1. The xcex1 light goes into the fiber 31, passes the WDM filter without reflection and propagates in the fiber 31. The receiving light of a wavelength xcex2 propagates in the reverse direction in the fiber 31. The xcex2 is reflected by the WDM 34. The reflected xcex2 goes upward, enters the PD 33 via the bottom, arrives at the light sensing region 35 and produces photocurrent. The transmitting xcex1 light and the receiving xcex2 light simultaneously make their ways in the fiber bidirectionally. Imperfection of the WDM filter 34 allows a part of the xcex1 light to leak into the PD. Besides the light via the WDM, stray xcex1 light which is scattered in random directions in the housing 30 shoots the PD at various angles. The leak xcex1 via WDM and the scattered stray xcex1 induce serious crosstalk in the PD. The LD/PD module of FIG. 5 sometimes inserts an extra multilayered filter between the WDM 34 and the PD 33.
Simultaneous bidirectional optical communications are annoyed at the crosstalk from the LD to the PD caused by the imperfection of the PD and the random scattering. The above-mentioned multilayered filter 5 is added on the bottom of the PD in the PD module of FIG. 1 for killing the crosstalk. The module of FIG. 5 also adds an extra multilayered filter between the WDM and the PD for annihilating the crosstalk.
The dielectric multilayered filter has strong incidence angle dependence. The filter has high selectivity for the light shooting at the designed incidence angle with high efficiency. The filter is incompetent for the light with an incidence angle different from the designed angle. In the case of a filter for orthogonal incidence angle, the filter can reflect the orthogonally incidence unnecessary xcex1. The filter, however, admits a part of unnecessary xcex1 which shoots the filter with an oblique incidence angle. A. part of the none-orthogonally incidence xcex1 passes the filter. Stray xcex1 light having random incidence angles easily penetrates the filter. The strong incidence angle dependence deprives the multilayered filter of the power of eliminating all the unnecessary xcex1. The incidence angle dependence is one of the most serious defects of the dielectric multilayered filters.
Another drawback of the dielectric multilayered filters is that the filter repulses unnecessary light by reflection. Reflected light survives in the package and is reflected in the package. Multiple reflection makes stray light in the package. The stray light disturbs the filtering function of the multilayered filter. The alternative of the dielectric multilayered filter is reflection or passage. The reflection is imperfect for the meaning of annihilating unnecessary light. In the design of the dielectric multilayered filter, the refractive indexes have only real parts. The imaginary parts of the refractive indexes are deemed to be constant against a change of wavelengths. The imaginary parts of the refractive index mean absorption. However the imaginary part of the refractive index of dielectrics has, in general, poor wavelength dependence. The change of the imaginary parts by a wavelength variation is nearly zero.
One purpose of the present invention is to provide a submount intervening between a package and a PD and having a function of eliminating unnecessary light.
Another purpose of the present invention is to provide a submount intervening between a package and a PD and having a function of preventing leak LD light or stray LD light from going into the PD of an LD/PD module.
A further purpose of the present invention is to provide a submount intervening between a package and a PD and having a function of eliminating unnecessary light with arbitrary incidence angles unlike the dielectric multilayered filter.
A further purpose of the present invention is to provide a submount having high heat resistance.
A further purpose of the present invention is to provide a submount suitable for facilitating mounting of a PD.
The optical part (submount) of the present invention is a passive chip base inserted between a metal case and a chip which includes a semiconductor or insulator substrate which is transparent to necessary light and at least one epitaxially-grown wavelength selective absorption films formed on at least one surface of the substrate which absorbs unnecessary light.
The wavelength of the unnecessary light is denoted by xcex1 and the wavelength of the necessary light is denoted by xcex2. The submount of the present invention is made by forming epitaxially wavelength selective absorption films which absorb the xcex1 light upon a transparent substrate which allows the xcex2 light to pass.
All the conventional submounts are opaque. In contradiction to the prior submounts, he substrate of the submount of the present invention is transparent. The wavelength elective absorption layer is transparent to the necessary xcex2 but opaque to the unnecessary xcex1. The chip base of the present invention allows only xcex2 to pass through but annihilates xcex1. The epitaxially-grown wavelength selective absorption films give the present invention such an asymmetric character of cutting xcex1 and admitting xcex2. xcex1-opacity and xcex2-transparency is a conspicuous feature of the present invention.
The submount of the present invention has common elements as a chip base which is inserted between a package (metal pattern) and a PD chip for insulating the PD from the package (metal pattern). The submount has fringing metallized layers on both surfaces for bonding the submount with a package and a PD chip by soldering. The central part of the submount should be free from the metallized layers for allowing xcex2 to penetrate. Thus, the metallized layers should coat only the fringe of the submount. When a resin adhesive is applied to one of the PD/submount bonding or the submount/case bonding, the metallized layer can be omitted for the surface for the resin adhesion. The central part without the metallized layer is blank or coated with an antireflection film.
The transparent substrate can be made from semiconductors, glass (amorphous materials), ceramics or dielectrics which are transparent to the object light xcex2. Semiconductors are the most suitable materials.
The wavelength selective absorption film can be also made from semiconductors, dielectrics, amorphous materials which have the required wavelength selectivity for absorbing xcex1 but admitting xcex2. Semiconductors are the most suitable to the wavelength selective absorption film. An appropriate pair is a semiconductor transparent substrate and semiconductor wavenumber selective absorption films.
The band gap of the wavelength selective absorption film should be determined by the wavelengths xcex1 and xcex2. The concept of the band gap can be defined to semiconductor, dielectrics and amorphous materials. In the case of semiconductors or dielectrics, both a polycrystal and a single crystal are available, because they have the same band gap. The absorption edge wavelength xcexg which corresponds to the band gap Eg by the relation of xcexg=hc/Eg, where h is Planck constant and c is light velocity in vacuum. The wavelength selective absorption material should be chosen by the condition of xcex1 less than xcexg less than xcex2.
Light filtering function of the present invention is quite different from prevalent dielectric multilayer filters which pile two or three kinds dielectric films by turns but a bit akin to photodiodes. The present invention owes the wavelength selectivity to the band gap of the material. Since the submount is a passive part, the submount should dispense with the reverse bias which would produce a wide depletion layer at the pn-junction, would generate pairs of electron and hole, would attract holes to the p-region and electrons to the n-region and would make photocurrent in a photodiode. The submount of the present invention has no pn-junction, no reverse bias, no depletion layer and no photocurrent.
The submount lacks the reverse bias. The reverse bias on the pn-junction in a PD aims at making photocurrent. The photoelectric conversion of absorbing light and generating photocarriers (electrons and holes) occurs also in the submount without pn-junction and reverse bias so long as the light energy is larger than the bang gap. The photoelectric conversion is independent of the pn-junction and the reverse bias. The probability of the photoelectric conversion is free from the reverse bias.
The light having wavelength xcex1 shorter than xcexg is absorbed and is converted into pairs of holes and electrons in the wavelength selective absorption layer. Since no bias is applied to the wavelength selective absorption layer, thermal diffusion and thermal agitation are origins of motion. No concentration distribution appears in the layer. The thermal agitation is the origin of motion of the holes and electrons. The averages of velocities and displacement are zero for the thermal agitation. The holes and the electrons stay in the wavelength selective absorption layer. The holes soon collide with electrons, recombine with the electrons, make heat and vanish. A series of the above facts can be simply expressed by that the xcex1 light is absorbed in the wavelength s elective absorption layer. Since xcexg is shorter than xcex2, the wavelength selective absorption layer absorbs no xcex2 light. Thus, the wavelength selective absorption layer absorbs xcex1 but admits xcex2.
The next problem is how to determine the minimum thickness of the wavelength selective absorption layer. Absorption coefficient xcex1 is defined in an absorption material. exp(xe2x88x92xcex1) is an attenuation rate for a unit length propagation. xcex1 is zero for the light of a longer wavelength than the absorption edge wavelength xcexg. xcex1 is a positive definite for the light of a shorter wavelength than xcexg. When the light makes its way by a distance x, the light of a unit power is attenuated to exp(xe2x88x92xcex1x). The wavelength selective absorption layer of a thickness d can decrease unnecessary light (xcex1) to a ratio of exp(xe2x88x92xcex1d). When the required attenuation is determined to be 1/100 or 1/1000, the suitable thickness d of the wavelength selective absorption layer can be calculated.
For example, the InGaAsP of xcexg=1.42 xcexcm has xcex1=104 cmxe2x88x921 for 1.3 xcexcm. If the wavelength selective absorption layer is made by the InGaAsP of xcexg=1.42 xcexcm, a thickness of d=5 xcexcm gives an attenuation ratio less than 1/100.
The advantages of the present invention are described. Conventional submounts consist of a ceramic substrate, in particular, alumina (Al2O3) substrate and overall metallized layers coating both surfaces of the substrate. Both the ceramic and the metallized layers are opaque. Prior submounts are opaque. The present invention replaces the ceramic substrate by a transparent semiconductor/insulator substrate and replaces the overall metallized layer by fringing metallized layers. The present invention adds a wavelength selective absorption layer upon the transparent substrate. The wavelength selective absorption layer admits necessary light xcex2 but absorbs unnecessary light xcex1. The submount of the present invention allows necessary light xcex2 to pass but absorbs unnecessary light xcex1.
A WDM filter separates the receiving light from the transmitting light in an LD/PD module. The submount of the present invention lowers the crosstalk from the LD to the PD by complementing the separating function of the WDM. Although the submount is only a passive element, the effect is conspicuous.
However, someone may feel hesitant of making use of expensive semiconductor element for a submount (chip base). An increase of semiconductor wafer size decreases the cost per chip. The cost of substrate is not so high. The cost of epitaxially growing the wavelength selective absorption layer is also decreased by the matured wafer process. The semiconductor submounts of the present invention can be made in a manner in an apparatus similar to making LDs or PDs. The probability of diverting the semiconductor technology lowers the cost of fabrication of the submounts.
On the contrary, the dielectric multilayered filter is rather expensive, since the multilayer requires a number of repetitions of coating of different dielectric layers on a resin substrate. The complexity of the layer structure raises the cost.
The dielectric multilayered filter made upon a resin substrate has poor resistance against heat. The resin filter should be inserted into the module for avoiding heat attack after mounting the PD, the LD and other devices on the package. The submount of the present invention with high heat resistance facilitates the assembly of LD/PD modules.